Transition metal complexes of selected bidentate ligands containing both phosphorous and nitrogen groups which coordinate to certain early and late transition metals are active catalysts (sometimes in the presence of other compounds) for the polymerization of ethylene. Polyethylenes prepared by some of these catalysts under certain conditions are novel, being highly branched and apparently having exceptionally stiff polymer chains in solution.
Polyethylenes are very important items of commerce, large quantities of various grades of these polymers being produced annually for a large number of uses, such as packaging films and moldings. There are many different methods for making such polymers, including many used commercially, such as free radical polymerization to make low density polyethylene, and many so-called coordination catalysts such as Ziegler-Natta-type and metallocene-type catalysts. Each of these catalyst systems has its advantages and disadvantages, including cost of the polymerization and the particular structure of the polyethylene produced. Due to the importance of polyethylenes, new catalyst systems which are economical and/or produce new types of polyethylenes are constantly being sought.
WO98/40420 describes the use of certain late transition metal complexes of ligands containing phosphorous and nitrogen as ingredients in polymerization systems for olefins. Many of the ligands disclosed herein are different from those disclosed in this reference.
WO97/48735 generally describes the use of certain complexes of late transition metals are polymerizations catalysts for olefins. Among the ligands in these complexes are those which contain both phosphorous and nitrogen. None of the ligands described herein are specifically described in this reference.
Linear polyethylene is reported (J. Brandrup, et al., Ed., Polymer Handbook, 3rd Ed., John Wiley and Sons, New York, 1989, p. VII/6) to have a Mark-Houwink constant (xcex1) of about 0.6-0.7 in 1,2,4-trichlorobezene at 135xc2x0 C. No mention is made of any polyethylenes with higher Mark-Houwink constants.
This invention concerns a first process for the production of polyethylene, comprising the step of contacting, at a temperature of about xe2x88x92100xc2x0 C. to about +200xc2x0 C., ethylene and a Ti, Cr, V, Zr, Hf or Ni complex of a ligand of the formula 
wherein:
T is hydrocarbylene, substituted hydrocarbylene or xe2x80x94CR9R10xe2x80x94;
R2 is hydrogen, hydrocarbyl or substituted hydrocarbyl;
R3 and R4 are each independently hydrocabyl or substituted hydrocarbyl, provided that a carbon atom bound to a nitrogen atom has at least two other carbon atoms bound to it;
R5 and R6 are each independently hydrogen, hydrocarbyl or substituted hydrocarbyl;
R7 and R8 are each independently hydrocarbyl or substituted hydrocarbyl, provided that a carbon atom bound to a phosphorous atom has at least two other carbon atoms bound to it;
R9 and R10 are each independently hydrogen, hydrocarbyl or substituted hydrocarbyl;
R11 and R12 are each independently hydrocarbyl or substituted hydrocarbyl;
R13 is hydrocarbyl or substituted hydrocarbyl;
and provided that R2 and R9 taken together may form a ring.
Also disclosed herein is a second process for the production of polyethylene, comprising the step of contacting, at a temperature of about xe2x88x92100xc2x0 C. to about +200xc2x0 C., ethylene, a compound of the formula 
and:
(a) a first compound W, which is a neutral Lewis acid capable of abstracting Xxe2x88x92 and alkyl group or a hydride group from M to form WXxe2x88x92, (WR20)xe2x88x92 or WHxe2x88x92 and which is also capable of transferring an alkyl group or a hydride to M, provided that WXxe2x88x92 is a weakly coordinating anion; or
(b) a combination of second compound which is capable of transferring an alkyl or hydride group to M and a third compound which is a neutral Lewis acid which is capable of abstracting Xxe2x88x92, a hydride or an alkyl group from M to form a weakly coordinating anion;
wherein:
M is Ti, Cr, V, Zr, Hf or Ni;
each X is an anion;
n is an integer so that the total number of negative charges on said anion or anions is equal to the oxidation sate of M;
T is hydrocarbylene, substituted hydrocarbylene or xe2x80x94CR9R10xe2x80x94;
R2 is hydrogen, hydrocarbyl or substituted hydrocarbyl;
R3 and R4 are each independently hydrocarbyl or substituted hydrocarbyl, provided that a carbon atom bound to a nitrogen atom has at least two other carbon atoms bound to it;
R5 and R6 are each independently hydrogen, hydrocarbyl or substituted hydrocarbyl;
R7 and R8 are each independently hydrocarbyl or substituted hydrocarbyl, provided that a carbon atom bound to a phosphorous atom has at least two other carbon atoms bound to it;
R9 and R10 are each independently hydrogen, hydrocarbyl or substituted hydrocarbyl;
R11 and R12 are each independently hydrocarbyl or substituted hydrocarbyl;
R13 is hydrocarbyl or substituted hydrocarbyl;
and provided that R2 and R9 taken together may form a ring.
This invention also concerns a third process for the production of polyethylene, comprising the step of contacting, at a temperature of about xe2x88x92100xc2x0 C. to about +200xc2x0 C., ethylene and a compound of the formula 
wherein:
M is Ti, Cr, V, Zr, Hf or Ni;
n is an integer so that the total number of negative charges on said anion or anions is equal to the oxidation sate of M;
T is hydrocarbylene, substituted hydrocarbylene or xe2x80x94CR9R10xe2x80x94;
R2 is hydrogen, hydrocarbyl or substituted hydrocarbyl;
R3 and R4 are each independently hydrocabyl or substituted hydrocarbyl, provided that a carbon atom bound to a nitrogen atom has at least two other carbon atoms bound to it;
R5 and R6 are each independently hydrogen, hydrocarbyl or substituted hydrocarbyl;
R7 and R8 are each independently hydrocarbyl or substituted hydrocarbyl, provided that a carbon atom bound to a phosphorous atom has at least two other carbon atoms bound to it;
R9 and R10 are each independently hydrogen, hydrocarbyl or substituted hydrocarbyl;
R11 and R12 are each independently hydrocarbyl or substituted hydrocarbyl;
R13 is hydrocarbyl or substituted hydrocarbyl;
Z1 is hydride or alkyl or any other anionic ligand into which ethylene can insert;
Y is a neutral ligand capable of being displaced by ethylene or a vacant coordination site;
Q is a relatively non-coordinating anion;
P is a divalent polyethylene group containing one or more ethylene units; and
Z2 is an end group
and provided that R2 and R9 taken together may form a ring.
This invention also concerns a homopolyethylene which has a Mark-Houwink constant of about 1.0 or more when measured in 1,2,4-trichlorobenzene.
A structure drawn such as (V), 
simply means that the ligand in the square bracket is coordinated to the metal containing moiety, as indicated by the arrow. Nothing is implied about what atoms in the ligand are coordinated to the metal.
Herein, certain terms are used. Some of them are:
A xe2x80x9chydrocarbyl groupxe2x80x9d is a univalent group containing only carbon and hydrogen. If not otherwise stated, it is preferred that hydrocarbyl groups herein contain 1 to about 30 carbon atoms.
By xe2x80x9csubstituted hydrocarbylxe2x80x9d herein is meant a hydrocarbyl group which contains one or more substituent groups which are inert under the process conditions to which the compound containing these groups is subjected. The substituent groups also do not substantially interfere with the process. If not otherwise stated, it is preferred that substituted hydrocarbyl groups herein contain 1 to about 30 carbon atoms. Included in the meaning of xe2x80x9csubstitutedxe2x80x9d are heteroaromatic rings.
By xe2x80x9c(inert) functional groupxe2x80x9d herein is meant a group other than hydrocarbyl or substituted hydrocarbyl which is inert under the process conditions to which the compound containing the group is subjected. The functional groups also do not substantially interfere with any process described herein that the compound in which they are present may take part in. Examples of functional groups include halo (fluoro, chloro, bromo and iodo), ether such as xe2x80x94OR18 wherein R18 is hydrocarbyl or substituted hydrocarbyl. In cases in which the functional group may be near a cobalt or iron atom, such as R4, R5, R8, R12, R13, and R17 the functional group should not coordinate to the metal atom more strongly than the groups in compounds containing R4, R5, R8, R12, R13, and R17 which are shown as coordinating to the metal atom, that is they should not displace the desired coordinating group.
By bound to a nitrogen or phosphorous atom is meant a nitrogen or phosphorous atom explicitly shown in compound (V) or one of its complexes.
By an xe2x80x9calkyl aluminum compoundxe2x80x9d is meant a compound in which at least one alkyl group is bound to an aluminum atom. Other groups such as alkoxide, hydride, and halogen may also be bound to aluminum atoms in the compound.
By xe2x80x9cneutral Lewis basexe2x80x9d is meant a compound, which is not an ion, which can act as a Lewis base. Examples of such compounds include ethers, amines, sulfides, and organic nitrites.
By xe2x80x9ccationic Lewis acidxe2x80x9d is meant a cation which can act as a Lewis acid. Examples of such cations are sodium and silver cations.
By relatively noncoordinating (or weakly coordinating) anions are meant those anions as are generally referred to in the art in this manner, and the coordinating ability of such anions is known and has been discussed in the literature, see for instance W. Beck., et al., Chem. Rev., vol. 88 p. 1405-1421 (1988), and S. H. Stares, Chem. Rev., vol. 93, p. 927-942 (1993), both of which are hereby included by reference. Among such anions are those formed from the aluminum compounds in the immediately preceding paragraph and Xxe2x88x92, including R93AlXxe2x88x92, R92AlClXxe2x88x92, R9AlCl2Xxe2x88x92, and xe2x80x9cR9AlOXxe2x88x92xe2x80x9d, wherein R9 is alkyl. Other useful noncoordinating anions include BAFxe2x88x92 {BAF=tetrakis[3,5-bis(trifluoromethyl)phenyl]borate}, SbF6xe2x88x92, PF6xe2x88x92, and BF4xe2x88x92, trifluoromethanesulfonate, p-toluenesulfonate, (RfSO2)2Nxe2x88x92, and (C6F5)4Bxe2x88x92.
By an empty coordination site is meant a potential coordination site that is not occupied by a ligand. Thus if an ethylene molecule is in the proximity of the empty coordination site, the ethylene molecule may coordinate to the metal atom.
By a ligand that may add to ethylene is meant a ligand coordinated to a metal atom into which an ethylene molecule (or a coordinated ethylene molecule) may insert to start or continue a polymerization. For instance, this may take the form of the reaction (wherein L is a ligand): 
Preferred transition metals are Ti, Cr, V, and Ni, and Ni is especially preferred. It is believed that for the most part Ti, Cr, V and other early transition metals will give polyolefins with a xe2x80x9cnormalxe2x80x9d amount of branching. For a discussion of xe2x80x9cnormalxe2x80x9d branching in polyolefins see WO96/23010, which is hereby included by reference. As can be seen from the results herein use of the Ni (and other) complexes often results in polymers with xe2x80x9cabnormalxe2x80x9d amounts of branching.
Preferred groups in compounds (I) and (II) and their corresponding metal complexes are:
R3 is hydrocarbyl especially alkyl or alkyl or halogen substituted aryl, more especially alkyl containing 2 to 6 carbon atoms and 2,6-dialkylphenyl;
R4 is hydrogen or alkyl, hydrogen especially when R3 is alkyl or halogen substituted aryl;
R7 and R8 are independently saturated hydrocarbyl, especially alkyl or cycloalkyl containing 3 to 8 carbon atoms;
R5 and R6 are independently hydrogen or methyl, more preferably both hydrogen;
R13 is alkyl or halogen substituted aryl, especially 2,6-disubstituted phenyl which may optionally be substituted in other positions;
R11 and R12 are each independently hydrocarbyl or substituted hydrocarbyl, especially hydrocarbyl in which the carbon atom bound to the phosphorous atom is bound to at least 2 other carbon atoms;
T is xe2x80x94CHR14 xe2x80x94 wherein R14 is hydrogen or alkyl containing 1 to 6 carbon atoms, T is xe2x80x94CR9R10xe2x80x94, or T is o-phenylene;
R10 is hydrogen and R2 taken together form a ring, especially a carbocyclic ring.
The ring formed by R2 and R9 may be part of monocyclic ring system, or part of another type of ring system, such as a bicyclic ring system. Preferred groups when R2 and R9 taken together form a ring are 
xe2x80x94(CH2)3xe2x80x94 and xe2x80x94(CH2)4xe2x80x94.
Specific preferred compounds for (I) and (II), and their corresponding transition metal complexes, are: 
In these formulas and otherwise herein Bu is butyl, Cy is cyclohexyl, and Ph is phenyl.
In all the polymerization processes herein, the temperature at which the polymerization is carried out is about xe2x88x92100xc2x0 C. to about +200xc2x0 C., preferably about 0xc2x0 C. to about 150xc2x0 C., more preferably about 25xc2x0 C. to about 100xc2x0 C. The ethylene concentration at which the polymerization is carried out is not critical, atmospheric pressure to about 275 MPa being a suitable range for ethylene and propylene.
The polymerization processes herein may be run in the presence of various liquids, particularly aprotic organic liquids. The catalyst system, ethylene, and polyethylene may be soluble or insoluble in these liquids, but obviously these liquids should not prevent the polymerization from occurring. Suitable liquids include alkanes, cycloalkanes, selected halogenated hydrocarbons, and aromatic hydrocarbons. Hydrocarbons are the preferred solvent. Specific useful solvents include hexane, toluene, benzene, chloroform, methylene chloride, 1,2,4-trichorobenzene, p-xylene, and cyclohexane.
The catalysts herein may be xe2x80x9cheterogenizedxe2x80x9d by coating or otherwise attaching them to solid supports, such as silica or alumina. Where an active catalyst species is formed by reaction with a compound such as an alkylaluminum compound, a support on which the alkylaluminum compound is first coated or otherwise attached is contacted with the nickel compound precursor to form a catalyst system in which the active nickel catalyst is xe2x80x9cattachedxe2x80x9d to the solid support. These supported catalysts may be used in polymerizations in organic liquids, as described in the immediately preceding paragraph. They may also be used in so-called gas phase polymerizations in which the ethylene being polymerized are added to the polymerization as a gas and no liquid supporting phase is present.
It is known that certain transition metal containing polymerization catalysts including those disclosed herein, are especially useful in varying the branching in polyolefins made with them, see for instance WO96/23010, WO97/02298, WO98/30610 and WO98/30609, incorporated by reference herein for all purposes as if fully set forth. It is also known that blends of distinct polymers, that vary for instance in the properties listed above, may have advantageous properties compared to xe2x80x9csinglexe2x80x9d polymers. For instance it is known that polymers with broad or bimodal molecular weight distributions may be melt processed (be shaped) more easily than narrower molecular weight distribution polymers. Similarly, thermoplastics such as crystalline polymers may often be toughened by blending with elastomeric polymers.
Therefore, methods of producing polymers which inherently produce polymer blends are useful especially if a later separate (and expensive) polymer mixing step can be avoided. However in such polymerizations one should be aware that two different catalysts may interfere with one another, or interact in such a way as to give a single polymer.
In such a process the catalysts disclosed herein can be termed the first active polymerization catalyst. Monomers useful with these catalysts are those described (and also preferred) above.
A second active polymerization catalyst (and optionally one or more others) is used in conjunction with the first active polymerization catalyst. The second active polymerization catalyst may be another late transition metal catalyst, for example as described in previously incorporated WO96/23010, WO97/02298, WO98/30610, WO98/30609 and WO98/27124. Other useful types of catalysts may also be used for the second active polymerization catalyst. For instance so-called Ziegler-Natta and/or metallocene-type catalysts may also be used. These types of catalysts are well known in the polyolefin field, see for instance Angew. Chem., Int. Ed. Engl., vol. 34, p. 1143-1170 (1995), EP-A-0416815 and U.S. Pat. No. 5,198,401 for information about metallocene-type catalysts; and J. Boor Jr., Ziegler-Natta Catalysts and Polymerizations, Academic Press, New York, 1979 for information about Ziegler-Natta-type catalysts, all of which are hereby included by reference for all purposes. Many of the useful polymerization conditions for all of these types of catalysts and the first active polymerization catalysts coincide, so conditions for the polymerizations with first and second active polymerization catalysts are easily accessible. Oftentimes the xe2x80x9cco-catalystxe2x80x9d or xe2x80x9cactivatorxe2x80x9d is needed for metallocene or Ziegler-Natta-type polymerizations. In many instances the same compound, such as an alkylaluminum compound, may be used as an xe2x80x9cactivatorxe2x80x9d for some or all of these various polymerization catalysts.
Suitable catalysts for the second polymerization catalyst also include metallocene-type catalysts, as described in U.S. Pat. No. 5,324,800 and EP-A-0129368; particularly advantageous are bridged bis-indenyl metallocenes, for instance as described in U.S. Pat. No. 5,145,819 and EP-A-0485823. Another class of suitable catalysts comprises the well-known constrained geometry catalysts, as described in EP-A-0416815, EP-A-0420436, EP-A-0671404 and EP-A-0643066, and WO91/04257.
Finally, the class of transition metal complexes described in WO96/13529 can be used.
All of the above references are hereby included by reference for all purposes as if fully set forth.
In one preferred process described herein the first olefin(s) [the monomer(s) polymerized by the first active polymerization catalyst] and second olefin(s) [the monomer(s) polymerized by the second active polymerization catalyst] are identical, and preferred olefins in such a process are the same as described immediately above. The first and/or second olefins may also be a single olefin or a mixture of olefins to make a copolymer. Again it is preferred that they be identical particularly in a process in which polymerization by the first and second active polymerization catalysts make polymer simultaneously.
In some processes herein the first active polymerization catalyst may polymerize a monomer that may not be polymerized by said second active polymerization catalyst, and/or vice versa. In that instance two chemically distinct polymers may be produced. In another scenario two monomers would be present, with one polymerization catalyst producing a copolymer, and the other polymerization catalyst producing a homopolymer, or two copolymers may be produced which vary in the molar proportion or repeat units from the various monomers. Other analogous combinations will be evident to the artisan.
In another variation of this process one of the polymerization catalysts makes an oligomer of an olefin, preferably ethylene, which oligomer has the formula R70CHxe2x95x90CH2, wherein Rxcex6is n-alkyl, preferably with an even number of carbon atoms. The other polymerization catalyst in the process them (co)polymerizes this olefin, either by itself or preferably with at least one other olefin, preferably ethylene, to form a branched polyolefin. Preparation of the oligomer (which is sometimes called an xcex1-olefin) by a second active polymerization-type of catalyst can be found in previously incorporated WO96/23010 and WO99/02472.
Likewise, conditions for such polymerizations, using catalysts of the second active polymerization type, will also be found in the appropriate above mentioned references.
Two chemically different active polymerization catalysts are used in this polymerization process. The first active polymerization catalyst is described in detail above. The second active polymerization catalyst may also meet the limitations of the first active polymerization catalyst, but must be chemically distinct. For instance, it may have a different transition metal present, and/or utilize a different type of ligand and/or the same type of ligand which differs in structure between the first and second active polymerization catalysts. In one preferred process, the ligand type and the metal are the same, but the ligands differ in their substituents.
Included within the definition of two active polymerization catalysts are systems in which a single polymerization catalyst is added together with another ligand, preferably the same type of ligand, which can displace the original ligand coordinated to the metal of the original active polymerization catalyst, to produce in situ two different polymerization catalysts.
The molar ratio of the first active polymerization catalyst to the second active polymerization catalyst used will depend on the ratio of polymer from each catalyst desired, and the relative rate of polymerization of each catalyst under the process conditions. For instance, if one wanted to prepare a xe2x80x9ctoughenedxe2x80x9d thermoplastic polyethylene that contained 80% crystalline polyethylene and 20% rubbery polyethylene, and the rates of polymerization of the two catalysts were equal, then one would use a 4:1 molar ratio of the catalyst that gave crystalline polyethylene to the catalyst that gave rubbery polyethylene. More than two active polymerization catalysts may also be used if the desired product is to contain more than two different types of polymer.
The polymers made by the first active polymerization catalyst and the second active polymerization catalyst may be made in sequence, i.e., a polymerization with one (either first or second) of the catalysts followed by a polymerization with the other catalyst, as by using two polymerization vessels in series. However it is preferred to carry out the polymerization using the first and second active polymerization catalysts in the same vessel(s), i.e., simultaneously. This is possible because in most instances the first and second active polymerization catalysts are compatible with each other, and they produce their distinctive polymers in the other catalyst""s presence. Any of the processes applicable to the individual catalysts may be used in this polymerization process with 2 or more catalysts, i.e., gas phase, liquid phase, continuous, etc.
The polymers produced by this xe2x80x9cmixed catalystxe2x80x9d process may vary in molecular weight and/or molecular weight distribution and/or melting point and/or level of crystallinity, and/or glass transition temperature and/or other factors. For copolymers the polymers may differ in ratios of comonomers if the different polymerization catalysts polymerize the monomers present at different relative rates. The polymers produced are useful as molding and extrusion resins and in films as for packaging. They may have advantages such as improved melt processing, toughness and improved low temperature properties.
Hydrogen may be used to lower the molecular weight of polyethylene produced in the first or second processes. It is preferred that the amount of hydrogen present be about 0.01 to about 50 mole percent of the ethylene present, preferably about 1 to about 20 mole percent. The relative concentrations of ethylene and hydrogen may be regulated by their partial pressures.
Included herein within the definitions of all the polymerization processes are mixtures of starting materials that lead to the formation in situ of the transition metal compounds specified in all of the polymerization processes.
Some of the homopolyethylenes produced herein have exceptionally high Mark-Houwink constants. Most homopolyethylenes have such constants in the range of about 0.5 to 0.75, depending on the particular solvent and temperature used, as well as the degree of branching in the polyethylene. It is believed that xe2x80x9cGenerally, 0.5xe2x89xa6xcex1xe2x89xa60.8 for flexible chains, 0.8xe2x89xa6xcex1xe2x89xa61.0 for inherently stiff molecules (e.g. cellulose derivatives, DNA) and 1.0xe2x89xa6xcex1xe2x89xa61.7 for highly extended chains (e.g. polyelectrolytes in solutions of very low ionic strength).xe2x80x9d, quotation from P. A. Lovell in G. Allen, et al., Ed., Comprehensive Polymer Science, Vol. 1, Pergamon Press, Oxford, 1989, p. 190. Why these polyethylenes behave as extended chain molecules in solution is not understood, but it is suspected that the branching patterns in these polymers are different from those in other branched homopolyethylenes, see for instance WO96/23010.
The polyethylenes with high Mark-Houwink constants are especially useful as viscosity modifiers, and are also useful the uses outlined for highly branched polyethylenes in WO96/23010, which is hereby included by reference, such as bases for lubricating oils, and lubricating oil viscosity modifiers. These polymers may be made by polymerizing using catalysts such as Ib, Ic and Id, more preferably Ib, and especially using higher polymerization temperatures, such as temperatures above about 50xc2x0 C., especially above about 70xc2x0 C., with maximum polymerization temperatures as described above.
In the Examples the following abbreviations are used:
xcex1xe2x80x94Mark-Houwink constant
[xcex7]xe2x80x94intrinsic viscosity
DSCxe2x80x94Differential Scanning Calorimetry
GPCxe2x80x94Gel Permeation Chromatography
MIxe2x80x94melt index
MMAOxe2x80x94methylaluminoxane modified with isobutyl groups
Mnxe2x80x94number average molecular weight
Mwxe2x80x94weight average molecular weight
PMAO-IPxe2x80x94methylaluminoxane
TCBxe2x80x941,2,4-trichlorobenzene
THFxe2x80x94tetrahydrofuran
Tmxe2x80x94melting point
Melting points were determined by DSC, using a heating rate of 10xc2x0 C./min. The melting point was taken on the 2nd heat, and the peak of the melting endotherm was taken as the melting point.
Intrinsic viscosity was measured in TCB at a temperature of 135xc2x0 C.
Branching levels as measured by 1H and 13C NMR were determined as described in WO96/23010.
All measurements and calculations relating to Mark-Houwink constants and for measurement of intrinsic viscosity were done as follows:
Measurements were made using a Waters xe2x80x9c150-CV plusxe2x80x9d chromatograph (Waters Corp.) with four Shodex(copyright) KF-806M columns (made by Showa Denko K. K., available from Showa Denko America, Inc., 280 Park Ave., New York, N.Y. 10017 U.S.A.) operating at 135xc2x0 C. in TCB at a flow rate of 1 mL/min. Injection volume was 150 microliters at a concentration of 1.5 mg/mL. Narrow fraction polystyrene standards from Polymer Laboratories Inc. were used to develop the universal calibration. A Waters Millennium(copyright) 2020 data system with GPCV software (Waters Corp.), version 2.15.1, was used to acquire and process the data. Intrinsic viscosities were measured at 35xc2x0 C.
The Mark-Houwink constants of the intrinsic viscosity-molecular weight relationship were obtained from a fit of the lower molecular-weight portion of the good data region; however, because the relationship was found to be nearly linear throughout the entire distribution of all subject polymers, the reported constants described the relationship of the higher molecular-weight species as well.